1. Field of the Invention
The present invention relates to an image retrieval device for optically retrieving an image and obtaining binary image data of the image.
2. Description of the Related Art
An image scanners using a charge couple device (CCD) sensor or a contact type image sensor are examples of image retrieval devices for obtaining binary data or multi-valued digital image data by optically retrieving an image. Recently, to answer demands for ever increasingly higher resolution, a 12 dot/mm or 16 dot/mm resolution image scanner has been developed.
However, the resolution of retrieval portions used in facsimile machines is set based on a transmission standard of 8 dot/mm. In the United States, the relative standard is defined by the Federal Communications Commission (FCC) Part 68, RS-470. In Japan, the relative standard is defined by the International Telegraph and Telephone Consultative Committee (CCITT). Therefore, when a high resolution image scanner is used as an image retrieval portion of a facsimile machine, there is a need to reduce the number of pixels representing the image. However, it is very difficult to reduce the number of pixels while the image is still in its optical form.
Therefore, in conventional image retrieval devices, pixels of the analog image signal are first converted into binary or multi-valued digital image data. Then, the image pixels are thinned at a predetermined rate to change the total number of the pixels. For example, when an image is retrieved at a resolution of 12 dot/mm, then by thinning the number of pixels down to two thirds of the original total retrieved, the image can be converted into the reduced resolution of 8 dot/mm. Similarly, when the image is retrieved at a resolution of 16 dot/mm, then by reducing thinning the number of pixels down to half the original total retrieved, then the required resolution of 8 dot/mm can be obtained.